G-proteins (GTP binding regulatory proteins) are the transducing elements that transmit the actions of hormones, neurotransmitters, odorants, light, antigens, and a variety of chemical signals that interact with receptors on the external surfaces of all eukaryotic cells, from yeast to man. Based on actions of cross-linking agents and hydrodynamic properties, these proteins isolated from rat brain synaptoneurosomes behave as multimeric, polydisperse structures. They resemble in their temperature sensitivities and disaggregating effects of guanine nucleotides cytoskeletal proteins such as actin and tubulin. Four of the prominent G-proteins found in brain display similar but varying properties depending on the species. Assuming these findings with extracted materials apply to intact cells, a new con- cept of signal transduction has been developed. In a related study, 100 kDa GTPbinding proteins containing epitopes reactive with antibodies against classical transduction proteins have been discovered in the liver and characterized. Finally hormones acting through G proteins have been shown to regulate fluid phase endocytosis in rat adipocytes and in Chinese Hamster Cells transfected with genes for muscarinic receptors I and III. These studies reveal a new function of G-proteins and suggest that the processing of pinosomes may be an important route for the dissemination of environmental signals and adaptation to the constantly changing levels of these signals in the environment.